stream/demux: move remaining callbacks to their respective ops table
This MR moves the remaining callbacks of stream/demux: pf_read/
pf_block/pf_seek/pf_readdir and pf_demux into their operations table,
aiming at unifying all the callbacks under a unique place.
This is a follow-up to the introduction of typed controls callbacks
for stream and demux.
Like for the typed controls callbacks, if no operation is provided
(ie, stream_t/demux_t.ops is NULL) by a module, the legacy pf_* will be
used instead as a fallback.
This MR doesn't migrate any of the modules yet.